Roof bolts with base plate and anchor shells are used extensively in mining and tunneling operations to reinforce and support weak rock strata. The invention herein disclosed relates to such roof bolting.
When tunneling or mining minerals in soft strata, it is necessary to shore up the roof of any underground passageway. This is normally accomplished by "bolting" the roof strata, a process well known in the art.
The conventional roof bolt anchor shell assembly is generally disclosed in the U.S. Pat. No. 2,783,673 issued to W. J. Lewis, et al. While certain refinements have since been added, such as those disclosed in U.S. Pat. No. 3,022,700 issued to J. B. Dempsey, most of the presently available mine roof bolts employ a threaded wedge nut to advance down a threaded shank. As the threaded shank is rotated, the wedge nut engages and expands an outer expansion shell unit into the wall of a previously drilled bolt hole. This outer expansion creates compressive or lateral stress and compresses the strata between the anchor shell and the base plate of the bolt assembly, which provides a safer mine roof, less susceptible to sagging.
Currently available roof bolts, however, are deficient in that the maximum expansion of the expansion shell is limited by the radial distance between the inner edge of the wedge nut and the outer edge of the base of the wedge nut, while presenting a relatively large area to provide transverse (compressive) thrust. Such limited radial expansion of conventional expansion shells provides transverse force applied over a large area, rather than a concentration of force (pressure) in a small area which would best provide deep penetration of the strata of the bolt hole wall.
If deeper penetration affording a greater area for support of the longitudinal load can be provided, a more appropriate and safer vertical force can be applied to the mine roof, which makes the bolting process much more effective. One method to provide such deeper penetration is disclosed in U.S. Pat. No. 3,192,822 issued to A. H. Genter. However the Genter method generally requires too much strata to be broken up before adequate anchoring can be obtained, if at all. A better device to obtain deep penetration without breaking up too much strata is disclosed in my earlier U.S. Pat. No. 4,293,244, Pick Arm Anchor Assembly, which provides a decided advantage over conventional roof bolt expansion shell systems. However, the present invention yields even deeper penetration of the roof strata (particularly in soft to medium-hard strata such as shale)--even more than provided by my Pick Arm Anchor Assembly--and represents a substantial advance in the state of the roof bolting art.